Resistant pigweed: The ultimate monkey wrench

For many farmers, it's the ultimate monkey wrench — something that is not only making farming more expensive — but is even forcing some to rethink how they farm.

This monkey wrench is glyphosate-resistant pigweed, also known as Palmer amaranth.

Farmers who try to grow cotton where these weeds are present and uncontrolled can count on one end result.

"Game over," says Michael Patterson, an Alabama Cooperative Extension System weed scientist and Auburn University professor of agronony, who stresses that cotton simply will not grow where pigweed has gotten a sufficient toehold.

"Palmer pigweed can grow one to two inches a day — cotton can't do that," Patterson says.

In only a few weeks, a field of Palmer pigweed can stand 12 to 18 inches high amid cotton that has grown no more than 5 to 8 inches. The end result: A crop starved of the vital nutrients and moisture it needs to mature.

In some parts of this state, this weed already has forced growers to abandon the convenient weed control model that has provided dramatic cost savings within the last decade and to completely rethink how they control weeds.

The old system was built on Roundup Ready cotton varieties transgenically engineered to withstand the effects of over-the-top applications of the weed herbicide Roundup (glyphosate).

Following chemical applications to burn down the winter cover crop, growers merely had to plant the crop into the residue and apply a few post-emergence sprayings during the growing season to control weeds.

"It was the cheapest system you could possibly use," Patterson says.

But with the specter of resistant pigweed spreading throughout the South, growers are being forced to abandon this comparatively cheap and effective weed control approach and return to older, more expensive methods.

In neighboring Georgia, more producers have been forced to return to older herbicides and to re-introduce tillage practices that were abandoned years ago following the advent of Roundup Ready cotton.

The threat has prompted Patterson and other experts to search for other alternatives. One Brazilian approach appears to offer some relief.

Ryegrass planted into tilled soil is allowed to grow until as late as early May. Using a special roller, the rye is then pressed onto the ground and the spring crop planted into this high residue.

"Once this rye is flattened, you don't see any bare ground," Patterson says. "It's just a thick carpet of rye."

This thick mat provides a thick cover that tends to suppress pigweed germination.

Aside from this approach, growers also can use Liberty Link transgenic cotton that works a lot like the Roundup Ready approach.

But as Patterson cautions, "Liberty Link cotton (used in tandem with Ignite herbicide) is a tool, but no magic bullet."

Much like the Brazilian approach, it will still require application of soil residual herbicides to slow weed growth and allow cotton to grow.

Moreover, the system affords growers only a narrow window of opportunity to deal with the resistant weeds.

Ignite can kill resistant pigweed that is under 4 inches high.

"By the time that pigweed starts to grow, producers have only about three or four days to control it using Ignite," Patterson says.